According to the helpfile, see screenshot below fm = 0.3 is used in the case no value is filled in.
[attachment=916]

According to the NPR 3659 and the document 'Integraal ontwerp leidingen en riolen, Grondmechanische randvoorwaarden' from GeoDelft the following conclusion has been made and formulae is given (which is also given in the NPR 3659):
[attachment=917]

I have done some Excel calculations for fm, see screenshot below.
[attachment=919]

This means that for peat/clay soil the fm may be too conservative. It would be nice to use the formula for fm in the Soil Model Wizard, where the phi is averaged over range from ground level to top of pipeline in the case no value is given for fm.